1. Field of the Invention
This invention relates generally to apparatus and methods for evaluating formations traversed by a well borehole, and more particularly to a testing apparatus and method for determining formation characteristics and preventing contamination of tool inner mechanisms and sensors.
2. Description of the Related Art
In the oil and gas industry, formation testing tools have been used for monitoring formation pressures along a well borehole, obtaining formation fluid samples from the borehole and predicting performance of reservoirs around the borehole. Such formation testing tools typically contain an elongated body having an elastomeric packer that is sealingly urged against a zone of interest in the borehole to collect formation fluid samples in fluid receiving chambers placed in the tool.
Downhole multi-tester instruments have been developed with extensible sampling probes for engaging the borehole wall at the formation of interest for withdrawing fluid samples therefrom and measuring pressure. In downhole instruments of this nature it is typical to provide an internal piston, which is reciprocated hydraulically or electrically to increase the internal volume of a fluid receiving chamber within the instrument after engaging the borehole wall. This action reduces the pressure at the instrument formation interface causing fluid to flow from the formation into the fluid receiving chamber of the instrument.
During drilling of a borehole, a drilling fluid (“mud”) is used to facilitate the drilling process and to maintain a pressure in the borehole greater than the fluid pressure in the formations surrounding the borehole. This is particularly important when drilling into formations where the pressure is abnormally high: if the fluid pressure in the borehole drops below the formation pressure, there is a risk of blowout of the well. As a result of this pressure difference, the drilling fluid penetrates into or invades the formations for varying radial depths (referred to generally as invaded zones) depending upon the types of formation and drilling fluid used. The formation testing tools retrieve formation fluids from the desired formations or zones of interest, test the retrieved fluids to ensure that the retrieved fluid is substantially free of mud filtrates, and collect such fluids in one or more chambers associated with the tool. The collected fluids are brought to the surface and analyzed to determine properties of such fluids and to determine the condition of the zones or formations from where such fluids have been collected.
One feature that all such testers have in common is a fluid sampling probe. This may consist of a durable rubber pad that is mechanically pressed against the formation adjacent the borehole, the pad being pressed hard enough to form a hydraulic seal. Through the pad is extended one end of a metal tube that also makes contact with the formation. This tube (“probe”) is connected to a sample chamber that, in turn, is connected to a pump that operates to lower the pressure at the attached probe. When the pressure in the probe is lowered below the pressure of the formation fluids, the formation fluids are drawn through the probe into the well bore to flush the invaded fluids prior to sampling. In some prior art devices, a fluid identification sensor determines when the fluid from the probe consists substantially of formation fluids; then a system of valves, tubes, sample chambers, and pumps makes it possible to recover one or more fluid samples that can be retrieved and analyzed when the sampling device is recovered from the borehole.
A problem associated with typical formation test tools is contamination within the tool inner mechanisms and sensors and consequent failures associated with such contamination. Another problem is increased time required even when only formation pressure testing is desired. There is a need for a quick formation pressure test that does not require large formation fluid volume and that does not allow contaminants into the tool inner mechanisms and sensors.